Digital systems often include one or more integrated circuits (also referred to as “chips”) that are coupled to one or more substrates, such as a printed circuit boards, using one or more package substrates. The printed circuit board provides power to the integrated circuit. The package substrate includes a plurality of power conductors and a plurality of ground conductors to electrically connect the integrated circuit to the printed circuit board.
Additionally, the digital system can include a plurality of decoupling capacitors that are connected between the power and ground conductors to help stabilize the voltage delivered to the integrated circuits. For example, when there is a sudden change in the current drawn by the integrated circuit, decoupling capacitors provide a local source of charge so that the current can be supplied quickly without causing the voltage across the power and ground nodes to dip suddenly. Inadequate decoupling leads to excessive power supply noise causing signal integrity and EMC problems, and ultimately adversely influencing the reliability of the product.
Presently, decoupling capacitors are placed on the integrated circuit, on the package substrate, and/or on the printed circuit board. The ability for the on-chip circuitry to access the charge stored in these capacitors depends on the impedance offered by the interconnect path to these capacitors. The impedance path to the on-chip or on-package substrate capacitors is relatively short and is useful to supply charge during very high frequency current transients. Unfortunately, the on-chip and the on-package substrate capacitors use valuable space. There is space for the capacitors on the printed circuit board, but the impedance path is relatively high and hence is not as effective during high frequency switching.